Flexographic printing, with photopolymer plates is conducted in the following manner: the photosensitive layer of the flexographic element is imagewise exposed using ultraviolet radiation. Unexposed areas are washed off using a suitable solvent. Solvent is removed by evaporation and, if necessary, the surface is treated to remove tack. The plate is mounted on press using a pressure-sensitive double-coated adhesive tape which is wrapped around and affixed to the plate cylinder. Then the plate is wrapped around and affixed to the outer surface of the tape in intimate contact with the adhesive. If the plate is not sufficiently flexible, the plate will tend to separate, i.e., delaminate, from the adhesive causing poor printing and can completely separate from the plate cylinder to preclude printing. Thus, it is important that the plate be flexible enough so that it remains wrapped around and affixed to the cylinder.
A variety of photopolymerizable compositions, having varying degrees of toughness, have been used to make flexographic printing plates. These compositions can be categorized according to the type of solvent in which they are developed, i.e., either organic solvent soluble or water soluble and they usually comprise (1) an addition polymerizable, nongaseous ethylenically unsaturated monomer, (2) a photoinitiator or photoinitiating system activated by actinic radiation and 3) a thermoplastic, elastomeric polymeric binder comprising polymerized conjugated diene monomers. Photopolymerizable layers are taught in Plambeck, U.S. Pat. No. 2,760,863, Chen et al. U.S. Pat. No. 4,323,636, Toda et al., U.S. Pat. No. 4,045,231; Heinz et al., U.S. Pat. No. 4,320,188. These compositions all suffer from the same defect, namely, they do not impart a sufficient degree of flexibility to plates made from these compositions. Plates made from these photosensitive compositions have a tendency to unwrap from the cylinder to which they are affixed.
U.S. Pat. No. 4,323,636 teaches a photosensitive composition having a solvent-soluble, thermoplastic elastomeric block copolymer to provide photosensitive solvent soluble elements which are useful for making flexographic printing reliefs for letter press printing. Those block copolymers have at least two thermoplastic, nonelastomeric polymer blocks having a glass transition temperature above 25.degree. C. and between said thermoplastic, nonelastomeric blocks, an elastomeric polymer block having a glass transition temperature below 10.degree. C. It is also taught that the monomer should be compatible with either block of the copolymer, and preferably the elastomeric block, in order to secure an essentially non-light scattering mixture. Other photopolymerizable compositions containing elastomeric block copolymers useful for preparing flexographic relief printing plates are taught in U.S. Pat. Nos. 4,430,417 and 4,045,231.
Photopolymerizable elements and processes for their use in preparing relief printing plates are well known in the art: U.S. Pat. No. 2,760,863, U.S. Pat. No. 3,556,791, U.S. Pat. No. 3,798,035, U.S. Pat. No. 3,825,430 and U.S. Pat. No. 3,951,657.